The present invention relates to oscillators which generate radio frequency signals. Specifically, a crystal oscillator for low power applications is described having improved long term and short term frequency stability.
Crystal oscillators have been used for many years as the signal generators for generating frequency stable radio frequency signals. The proliferation of low power communication devices, such as cellular telephones, has placed additional constraints on oscillator design since a major objective of the design is to preserve battery life. In applications such as the cellular telephones, a frequency synthesizer may typically be used to generate a local oscillator signal for the superhetrodyne receiver within the telephone. The frequency synthesizer as is well known relies upon a stable source of radio frequency signals from which to synthesize other signals needed in the frequency conversion process for the telephone receiver. The source of radio frequency signals must have good long term stability, i.e., the frequency drift must be within a tolerable range, as well as a high short term stability constituting the noise floor for the radio frequency signal.